Diabetes insipidus

Question 1

What is diabetes insipidus?

Answer 1

A disorder of inadequate secretion of a or insensitivity to vasopressin (ADH) leading to hypotonic polyuria

Question 2

What is the aetiology of diabetes insipidus?

Answer 2

• Failure of ADH secretion by the posterior pituitary (central/cranial) or insensitivity of the collecting duct to ADH (nephrogenic) - Water channels (aquaporins) fails to activate and the luminal membrane of the collecting duct remained impermeable to water - This results in large volume hypotonic urine and polydypsia

Question 3

What causes central diabetes insipidus?

Answer 3

• Idiopathic - Tumours e.g. pituitary tumours - Infiltrative e.g. sarcoidosis - Infection e.g. meningitis - Vascular e.g. aneurysms, Sheehan syndrome - Trauma (e.g. head injury, neurosurgery, DIDMOAD

Question 4

What causes nephrogenic diabetes insipidus?

Answer 4

• Idiopathic - Drugs e.g. lithium - Post-obstructive uropathy - Pyelonephritis - Pregnancy - Osmotic diuresis (e.g. diabetes mellitus)

Question 5

What is the epidemiology of diabetes insipidus?

Answer 5

• Depends on aetiology, but median age of onset is 24 yrs

Question 6

What are the presenting symptoms of diabetes insipidus?

Answer 6

• Polyuria, nocturia and polydipsia (excessive thirst) - Enuresis and sleep disturbances in children - Other symptoms depend on the aetiology

Question 7

What are the signs of diabetes insipidus on examination?

Answer 7

• Cranial diabetes insipidus has few signs if patients drink adequate fluids - Urine output is often more than 3L/24h - If fluid intake is less than fluid output, signs of dehydration may be present (e.g. tachycardia, reduced tissue turgor, postural hypotension, dry mucous membranes) - Signs of the cause (e.g. visual field defect)

Question 8

What are the investigations for diabetes insipidus

Answer 8

• Blood: U&E and Ca++ (Na+ may be rise secondary to dehydration) Raised plasma osmolality. Decreased urine osmolality - Water deprivation test: Water is restricted for 8h. Plasma and urine osmolality are measured every hour over the 8h. Weigh the patient hourly to monitor the level of dehydration; stop the test if the fall in body weight more than 3%. Desmopressin is given after 8h and urine osmolality is measured

Question 9

How is diabetes insipidus managed?

Answer 9

• Treat the identified cause if possible - Cranial diabetes insipidus: Desmopressin (DDAVP), a vasopressin analogue, can be given to potentiate effects of residual vasopressin - Nephorogenic diabetes insipidus: Sodium and/or protein restriction may help polyuria. Thiazine diuretics

Question 10

What are the complications of diabetes insipidus?

Answer 10

• Hypernatraemicd dehydration - Excess desmopressin therapy may cause hyponatraemia

Question 11

What is the prognosis of diabetes insipidus?

Answer 11

• Variable depending on cause - Cranial diabetes insipidus may be transient following head trauma - Cure of cranial or nephrogenic insipidus may be possible on removal of cause e.g. tumour resection, drug discontinuation